Initiation of combustion in a subterranean petroleum oil reservoir



United States Patent INITIATION 0F COMBUSTION IN A SUBTER RANEANPETROLEUM OIL RESERVOIR Robert L. Koch, Dallas, Tex., assignor, by mesneassignments, to Socony Mobil Oil Company, Incorporated, a corporation ofNew York No Drawing. Application March 9, 1953 Serial No. 341,337

9 Claims. (Cl. 166-41) This invention relates to recovery of petroleumoil from subterranean reservoirs and relates more particularly to thecombustion method of recovering petroleum oil from subterraneanreservoirs.

Petroleum oil is generally recovered initially from most subterraneanreservoirs as a result of gas pressure or natural water drive forcingthe oil from the oil-bearing formation or reservoir to the producingwell and to the surface of the earth. As recovery of oil from thereservoir continues, the reservoir energy gradually decreases andfinally becomes insufiicient to force the oil to the surface of theearth, although a major portion of the oil originally in the reservoirremains therein. In other reservoirs, the reservoir energy, from thestandpoint of the characteristics of the petroleum oil or otherwise, maybe insufficient to force the oil from the reservoir to the producingwell. To recover oil from these reservoirs, pumping may be employed butwhen the rate of recovery by pumping falls to an uneconomically lowlevel, or pumping is inefiective, as for example where the viscosity ofthe oil is too high to eflect movement thereof by pumping, oil may oftenbe economically recovered by the employment of methods such as gas driveorwater drive. It has recently been proposed to recover oil fromreservoirs where these methods are uneconomical or ineffective bycombustion or burning of-a part of the oil in place in the reservoir,the combustion being supported by continuous injection of air or otheroxidizing medium through an input well or wells, whereby as a result ofdecreased viscosity and distillation and viscosity breaking, theunburned oil, along with distillation and viscosity breaking products,may be carried to and recovered from an output well or group of outputwells.

It has been proposed to initiate combustion of the oil in place in thereservoir by placing charcoal in the input well adjacent to thereservoir, the liner, if the well contains a liner, being perforatedwhere it contacts the reservoir, injecting air or other combustionsupporting medium through the input well and into the reservoir, andigniting the charcoal. It has also been proposed to initiate combustionof the oil in place in the reservoir by locating an electric heater inthe input well adjacent to the reservoir, the liner, if any, beingperforated, sup plying air or other combustion supporting medium throughthe input well and into the reservoir, and supplying electric current tothe heater. Combustion may be effected by these and similar methods butin many instances the combustion cannot be sustained. While conditionswithin the reservoir, as determined by analysis of core samples takenfrom the reservoir, and otherwise, and conditions of rate of flow of thecombustion supporting medium are favorable for the maintenance ofcombustion, shortly after combustion is first effected, theconcentration of combustion supporting medium in the eflluent gasesbegins to increase and increases until the eflluent gases havepractically the same composition as the combustion supporting medium,signifying cessation of combustion. Further, the pressure required tomaintain the 2,818,117 Patented Dec. 31, 1957 2 necessary or desiredrate of supply of combustion supporting medium to the reservoir beginsto increase toward excessively high values. Subsequent attempts toinitiate sustained combustion within the reservoir by the same methodsencounter the same result and many reservoirs are subject to beingregarded as unsuitable for treatment by the combustion process becauseof impossibility of sustaining combustion after initiation.

It is an object of this invention to extend the applicability of thecombustion process for recovery of oil from subterranean oil-containingreservoirs. It is another object of this invention to provide a methodfor initiating combustion within a subterranean petroleum reservoir. Itis another object of this invention to provide a method for maintainingcombustion within a subterranean petroleum oil reservoir aftercombustion has been first effected. These and other objects of theinvention will become apparent from the following description thereof.

In accordance with the invention, a subterranean oilcontaining reservoirhaving an input well leading thereto and an output well leadingtherefrom is supplied through the input well with combustion supportingmedium and with heat to effect combustion within the reservoir, andthereafter the supply of combustion supporting medium is maintainedwhile the supply of heat is alternately discontinued and continued untilsustained combustion is effected within the reservoir.

In the practice of the invention, the reservoir is heated at the inputWell to a sufiicient temperature and supplied with combustion supportinggas in sufiicient amount to effect combustion of the oil Within thereservoir. As heat and combustion supporting gas are supplied to thereservoir, the ratio between the rate at which the combustion supportinggas is supplied to the reservoir and the pressure at which the gas issupplied to the reservoir will begin to increase but will thereafterdecrease. Heat and combustion supporting gas are continued to besupplied to the reservoir at least until the rate-pressure ratio of thegas decreases to below its original value and at this time the supply ofheat to the reservoir is discontinued. However, the supply of combustionsupporting gas is continued. With the supply of heat discontinued andthe supply of combustion supporting gas continued, the rate-pressureratio of the gas will thereafter begin to increase, and the heat supplyis discontinued at least until the rate-pressure ratio of the gas beginsto increase. After the rate-pressure ratio of the gas begins toincrease, heat is again supplied to the reservoir and the rate-pressureratio of the gas, which was previously increasing, will again begin todecrease. Heat is continuously supplied to the reservoir at least untilthe rate-pressure ratio of the gas begins to decrease and, at this time,the supply of heat is again discontinued. The rate-pressure ratio of thegas will thereafter begin to increase and the supply of heat isdiscontinued at least until the rate-pressure ratio of the gas begins toincrease and thereafter heat is again supplied to the reservoir. If therate-pressure ratio of the gas does not begin to decrease, sustainedcombustion will have been effected, and the supply of heat isdiscontinued. However, if the rate-pressure ratio of the gas begins todecrease when the supply of heat is continued, the procedure ofdiscontinuing the supply of heat, and again supplying heat, whilemaintaining the supply of combustion supporting gas, is repeatedthereafter until such time that the rate-pressure ratio of the gas nolonger decreases when heat is being supplied to the reservoir signifyingthat combustion is proceeding satisfactorily in the reservoir. Thesupply of heat is then discontinued and combustion will be maintained bysupplying combustion supporting gas to the reservoir.

At a minimum, two cycles of discontinuing the supply of heat to thereservoir and thereafter supplying heat to the reservoir will berequired in order to initiate sustained combustion, i. e., combustionwhich can be maintained by supplying combustion supporting gas to thereservoir. However, a greater number of cycles may be required. Nogeneral rule can be given as to the number of cycles that will berequired since the number of cycles required will depend upon reservoirconditions, the rate and pressure at which the combustion supporting gasis supplied, and the extent to which the rate-pressure ratio of the gasis allowed to decrease before thereafter supplying heat. In each case,therefore, the cycle of discontinuing the supply of heat and thereaftersupplying heat is repeated at least twice, and as many more times as maybe required, to initiate sus tained combustion.

Prior to the time sustained combustion is effected, the rate-pressureratio will increase during the time that heat is not being supplied tothe reservoir and will decrease during the time that heat is beingsupplied to the reservoir and there will be a lag between the time thatthe supply of heat is discontinued or continued and the time that therate-pressure ratio changes its direction from an increase to a decreaseor from a decrease to an increase. In each cycle, heat is supplied tothe reservoir at least until the rate-pressure ratio of the gas beginsto decrease, and the supply of heat is discontinued until at least therate-pressure ratio of the gas begins to increase. Heat may becontinuously supplied to the reservoir, after the rate-pressure ratio ofthe gas has begun to decrease, until the rate-pressure ratio of the gashas decreased to any desired extent below its previous maximum value.For example, the heat may be supplied to the reservoir until therate-pressure ratio of the gas has decreased to about one-half itsprevious maximum value before the supply of heat is discontinued.Generally, the extent to which the rate-pressure ratio of the gas ispermitted to decrease before the supply of heat is discontinued willdepend upon the economics of supplying combustion supporting gas to thereservoir at increased pressure and the capacity of the equipmentsupplying the gas. Similarly, the supply of heat to the reservoir may bediscontinued, after the rate-pressure ratio of the gas has begun toincrease, until the rate-pressure ratio of the gas has increased to anydesired extent above its previous minimum value, as for example, twiceits previous minimum value. However, it is preferred not to permit therate-pressure ratio of the gas to increase above the rate-pressure ratioof the gas at the time the heat and combustion supporting gas is firstsupplied to the reservoir in order that excessive cooling of thereservoir, involving waste of heat in the next heat-supplying step ofthe cycle, is avoided.

Heat is supplied to the reservoir by any suitable means that can becontrolled with respect to discontinuing and continuing the supply ofheat to the reservoir when desired. lt is preferred, for supplying heatto the reservoir, to employ a heater positioned in the input wellalongside the reservoir. Suitable types of heaters include electricheaters and gas-fired heaters, but an electric heater is preferred. Theheater need not be positioned alongside the reservoir but may bepositioned in the input well at any point above the reservoir sincesufficient heat may be supplied to the reservoir by reason of thecombustion supporting gas passing over the heater prior to entering thereservoir. Similarly, heat may be supplied to the reservoir by heatingthe combustion supporting gas at the surface of the earth prior toentering the input well.

Temperatures required to effect combustion within the reservoir dependupon the character of the petroleum oil within the reservoir but atemperature of 700 F. will ordinarily effect combustion. However, highertemperatures may be employed, as for example, 1400 F. Lowervtemperatures, suchas 400 F. or 500 R, will often be satisfactory. Thesetemperatures may be readily obtained with electric heaters and gas-firedheaters, and where heat is supplied to the reservoir by heating thecombustion supporting gas at the surface of the ground prior to enteringthe input well, the gas may be readily heated to these and highertemperatures in electrically operated or gas-, liquid-, or solidfuel-fired heat exchangers or by other means.

The combustion supporting gas is preferably air. However, any gascapable of supporting combustion within the reservoir may be employed.For example, oxygen. oxygen-enriched air, air admixed with inert gas toreduce the proportion of oxygen, oxygen admixed with inert gas, and fluegases containing oxygen may be employed.

The pressures employed for supplying the combustion supporting gas tothe reservoir must be sufficient to obtain the desired rate of supply ofcombustion supporting gas to the reservoir. Pressure drop of thecombustion supporting gas, of course, will occur in the reservoir andthe pressure drop will depend upon the permeability of the formation andupon other factors such as the fluid saturations of the reservoir.Accordingly, no general rule can be given with respect to the pressuresto be employed for supplying the combustion supporting gas. However,pressures can be selected in operation of the process to give thedesired rate of supply of the combustion supporting gas to the reservoirand the amount of combustion supporting gas required may be determinedfrom analysis of core samples taken from the reservoir and otherwise.

The following example will be illustrative of the invention. In theexample, the times referred to indicate the total accumulated time fromthe beginning of operation.

A subterranean petroleum oil reservoir located in Oklahoma andcontaining Mid-Continent base crude and having an input well leadingthereto and an output well leading therefrom was heated by means of anelectric heater positioned in the input well alongside the reservoir andair was pumped into the input well and into the reservoir. The.temperature of the heater was 500 F. and the air was supplied to theinput well at a rate of 510 standard cubic feet per hour at a pressureof pounds per square inch gauge, the rate-pressure ratio being 4.9. Airand heat were continued to be supplied to the reservoir and, after 15hours, carbon dioxide appeared in a concentration of 0.5 percent in theefliuent gases from the output well. During this 15-hour period, therate-pressure ratio of the gas increased to 7.5 with the air pressureincreasing to pounds per square inch gauge and the air rate increasingto 895 standard cubic feet per hour. The temperature of the heater hadincreased to 980 F. With continued supply of heat and air to thereservoir, the concentration of carbon dioxide in the effluent gasesfrom the output Well increased but later decreased and the concentrationof oxygen in the efiiuent gases decreased but later increased indicatingcessation of combustion. Further, the air rate decreased despite anincrease in the pressure until at 84 hours the air rate had decreased to245 standard cubic feet per hour and the pressure had increased topounds per square inch gauge, the rate-pressure ratio being 1.6. At

- this point, the electric heater was turned off but the supply of airwas continued.

After the supply of heat was discontinued, the ratepressure ratio slowlyincreased and at 115 hours the air rate had increased to 505 standardcubic feet per hour at a pressure of 14-6 pounds per square inch gauge,the rate-pressure ratio being 3.5. At this time, the electric heater wasturned on. At 118 hours, the concentration of carbon. dioxide in theeffluent gases began to increase, the concentration of oxygen in theefiiuent gases began t-o:decrease,'=and the air rate. had decreased to295 stand .ard cubic feet per hour and the pressure had increased to 150pounds per square inch gauge, the rate-pressure ratio being 2.0. Theaverage temperature of the heater betweten 115 hours and 118 hours was610 F. Heat was continuously supplied to the reservoir until 205 hours,at which time the air rate was 480 standard cubic feet per hour, thepressure was 150 pounds per square inch gauge, the rate-pressure ratiobeing 3.2, and the concentration of carbon dioxide in the efiluent gaseswas 6.5 percent. The average temperature of the heater between 118 hoursand 205 hours was 1050 F. At 205 hours the electric heater was turned edto determine whether combustion would be self-sustaining with air beingsupplied to the reservoir. However, with the supply of heatdiscontinued, the concentration of carbon dioxide in the efiluent gasslowly began to drop and at 252 hours had decreased to zero indicatingthat combustion was not self-sustaining. At this time, the air rate was1045 standard cubic feet per hour and the pressure was 150, therate-pressure ratio being 7.0. The heater was then turned on for thethird time at 259 hours and at 261 hours the carbon dioxideconcentration in the effluent gases began to increase. At 261 hours, theair rate was 815 cubic feet per hour, the pressure was 150 pounds persquare inch gauge, and the rate-pressure ratio was 5.4.

As heat was continued to be supplied to the reservoir for the thirdtime, the rate-pressure ratio dropped to a minimum of 2.0 and then beganto increase and at 451 hours the rate-pressure ratio was 3.2 with therate being 480 standard cubic feet per hour and the pressure 150 poundsper square inch gauge. The carbon dioxide concentration in the effluentgases was 9 percent. The average temperature of the heater between 259hours and 451 hours was 1250 F. The electric heater was turned oif forthe third time, again to determine whether combustion wasself-supporting, but the carbon dioxide concentration in the eifluentgases began to decrease and the rate-pressure ratio increased,indicating that combustion had again ceased.

At 1,013 hours, combustion having ceased, the electric heater was turnedon for the fourth time and the carbon dioxide concentration began toincrease. At 1,064 hours, the rate-pressure ratio was 26.2, the air ratebeing 3,150 standard cubic feet per hour and the pressure 120 pounds persquare inch gauge. The temperature of the heater was 1,340 F.

At 1,202 hours the electric heater was turned oif and during the timebetween 1,064 and 1,202 hours the carbon dioxide concentration in theeffiuent gases and the air rate were increasing. At the time theheaterwas turned off, the rate-pressure ratio was 27.7, the air ratebeing 3,050 standard cubic feet per hour, and the pressure being 110pounds per square inch gauge, and the concentration of carbon dioxide inthe eflluent gases was 14 percent. Thereafter, there were no increasesin the rate-pressure ratio, the concentration of carbon dioxide in theeffluent gases did not increase, and combustion was maintained only bysupplying air to the reservoir.

Having thus described my invention, it will be understood that suchdescription has been given by way of illustration and example and not byway of limitation, reference for the latter purpose being had to theappended claims.

I claim:

1. A process of initiating combustion in a subterranean petroleum oilreservoir comprising supplying said reser-. voir through an input wellleading thereto with air under pressure and with heat to effectcombustion of petroleum oil within said reservoir, thereaftermaintaining the supply of air to said reservoir, continuing the supplyof heat to said reservoir until the rate-pressure ratio of the airdecreases, next discontinuing the supply of heat to said reservoir untilthe rate-pressure ratio of the air increases, thereafter continuing thesupply of heat until the ratepressure ratio of the air decreases, andthen alternately discontinuing the supply of heat and continuing thesupply of heat with change in the rate-pressure ratio of the air untilsustained combustion is effected within said reser- 2. A process ofinitiating combustion of a combustible material in a subterraneanformation containing said combustible material comprising passing underpressure into said subterranean formation through an input well leadingthereto a combustion supporting medium for said combustible material andsupplying heat to said subterranean formation to effect combustion ofsaid combustible material, thereafter continuing passage of saidcombustion. supporting medium into said subterranean formation,continuing the supply of heat to said subterranean formation until therate-pressure ratio of said combustion sup porting medium decreases,next discontinuing the supply of heat to said subterranean formationuntil the ratepressure ratio of said combustion supporting mediumincreases, thereafter continuing the supply of heat to said subterraneanformation until the rate-pressure ratio of said combustion supportingmedium decreases, and then alternately discontinuing the supply of heatto said subterranean formation until the rate-pressure ratio of saidcombustion supporting medium increases and continuing the supply of heatto said subterranean formation until the rate-pressure ratio of saidcombustion supporting medium decreases until sustained combustion iseffected within said subterranean formation.

3. A process of initiating combustion of a combustible material in asubterranean formation containing said combustible material comprisingpassing air under pressure into said subterranean formation through aninput well leading thereto and supplying heat to said subterraneanformation to effect combustion of said combustible material, thereaftercontinuing passage of said air into said subterranean formation,continuing the supply of heat to said subterranean formation until therate-pressure ratio of said air decreases, next discontinuing the supplyof heat to said subterranean formation until the rate-pressure ratio ofsaid air increases, thereafter continuing the supply of heat to saidsubterranean formation until the ratepressure ratio of said airdecreases, and then alternately discontinuing the supply of heat to saidsubterranean formation until the rate-pressure ratio of said airincreases and continuing the supply of heat to said subterraneanformation until the rate-pressure ratio of said air decreases untilsustained combustion is effected within said subterranean formation.

4. A process of initiating combustion of a combustible material in asubterranean formation containing said combustible material comprisingpassing under pressure into said subterranean formation through an inputwell leading thereto a combustion supporting medium for said combustiblematerial and supplying heat to said subterranean formation to effectcombustion of said combustible material, thereafter continuing passageof said combustion supporting medium into said subterranean formation,continuing the supply of heat to said subterranean formation until therate-pressure ratio of said combustion supporting medium decreases belowits original value, next discontinuing the supply of heat to saidsubterranean formation until the rate-pressure ratio of said combustionsupporting medium increases, thereafter continuing the supply of heat tosaid subterranean formation until the rate-pressnre ratio of saidcombustion supporting medium decreases, and then alternatelydiscontinuing the supply of heat to said subterranean formation untilthe rate-pressure ratio of said combustion supporting medium increasesand continuing the supply of heat to said subterranean formation untilthe rate-pressure ratio of said combustion supporting medium decreasesuntil sustained combustion is effected within said subterraneanformation.

5. A process of initiating combustion of a combustible material in asubterranean formation containing said combustible material comprisingpassing under pressure into said subterranean formation through an inputwell leading thereto a combustion supporting medium for said combustiblematerial and supplying heat to said subterranean formation to effectcombustion of said combustible material, thereafter continuing passageof said combustion supporting medium into said subterranean formation,continuing the supply of heat to said subterranean formation until therate-pressure ratio of said combustion supporting medium decreases belowits original value, next discontinuing the supply of heat to saidsubterranean formation until the rate-pressure ratio of said combustionsupporting medium has increased above said original value, thereaftercontinuing the supply of heat to said subterranean formation until therate-pressure ratio of said combustion supporting medium decreases belowsaid original value, and then alternately discontinuing the supply ofheat to said subterranean formation until the rate-pressure ratio ofsaid combustion supporting medium increases above said original valueand continuing the supply of heat to said subterranean formation untilthe ratepressure ratio of said combustion supporting medium decreasesbelow said original value until sustained combustion is effected Withinsaid subterranean formation.

6. A process of initiating combustion of a combustible material in asubterranean formation containing said combustible material comprisingpassing under pressure into said subterranean formation through an inputWell leading thereto a combustion supporting medium for said combustiblematerial and supplying heat to said subterranean formation to effectcombustion of said combustible material, thereafter continuing passageof said combustion supporting medium into said subterranean formation,continuing the supply of heat to said subterranean formation until therate-pressure ratio of said combustion supporting medium decreases to atleast one-half its original value, next discontinuing the supply of heatto said subterranean formation until the rate-pressure ratio of saidcombustion supporting medium increases, thereafter continuing the supplyof heat to said subterranean formation until the rate-pressure ratio ofsaid combustion supporting medium decreases to at least one-half itsoriginal value, and then alternately discontinuing the supply of heat tosaid subterranean formation until the rate-pressure ratio of saidcombustion supporting medium increases and continuing the supply of heatto said subterranean formation until the rate-pressure ratio of saidcombustion supporting medium decreases to at least one-half its originalvalue until sustained combustion is effected within said reservoir. 7, Aprocess of initiating combustion of a combustible material in asubterranean formation containing said combustible material compresingpassing under pressure into said subterranean formation through an inputwell leading thereto a combustion supporting medium for said combustiblematerial and supplying heat to said subterranean formation to effectcombustion of said combustible material, thereafter continuing passageof said combustion supporting medium into said subterranean formation,

continuing the supply of heat to said subterranean formation until therate-pressure ratio of said combustion supporting medium decreases to atleast one-half its original value, next discontinuing the supply of heatto said subterranean formation until the rate-pressure ratio of saidcombustion supporting medium increases to at least twice its previousminimum value, thereafter continuing the supply of heat to saidsubterranean formation until the ratepressure ratio of said combustionsupporting medium decreases to at least one-half its original value, andthen alternately discontinuing the supply of heat to said subterraneanformation until the rate-pressure ratio of said combustion supportingmedium increases to at least twice its previous minimum value andcontinuing the supply of heat to said subterranean formation until therate-pressure ratio of said combustion supporting medium de- 8 v creasesto at least one-half its original value until sustained combustion iseffected within said subterranean formation.

8. A process of initiating combustion of a combustible material in asubterranean formation containing said combustible material and havingan input Well leading thereto comprising positioning a heater in saidinput Well at a point such that upon operation of said heater heat willbe supplied to combustion supporting medium passed under pressure intosaid subterranean formation through said input well, passing underpressure into said subterranean formation through said input well acombustion support ing medium and operating said heater to supply heatto said combustion supporting medium in an amount to bring itstemperature above the ignition temperature of said combustible materialupon entering said subterranean formation, thereafter continuing passageof said combustion supporting medium into said subterranean formation,continuing operating said heater to supply heat to said combustionsupporting medium in an amount to bring its temperature above theignition temperature of said combustible material upon entering saidsubterranean formation until the rate-pressure ratio of said combustionsupporting medium decreases, next discontinuing operating said heater tosupply heat to said combustion supporting medium in an amount to bringits temperature above the ignition temperature of said combustiblematerial upon entering said subterranean formation until therate-pressure ratio of said combustion supporting medium increases,thereafter operating said heater to supply heat to said combustionsupporting medium in an amount to bring its temperature above theignition temperature of said combustible material upon entering saidsubterranean formation until the rate-pressure ratio of said combustionsupporting medium decreases, and then alternately discontinuingoperating said heater to supply heat to said combustion supportingmedium in an amount to bring its temperature above the ignitiontemperature of said combustible material upon entering said subterraneanformation until the rate-pressure ratio of said combustion supportingmedium increases and operating said heater to supply heat to saidcombustion supporting medium in an amount to bring its temperature abovethe ignition temperature of said combustible material upon entering saidsubterranean formation until the rate-pressure ratio of said combustionsupporting medium decreases until sus tained combustion is effectedwithin said subterranean formation.

9. A process of initiating combustion of a combustible material in asubterranean formation containing said combustible material and havingan input well leading thereto comprising positioning a heater in saidinput well at a point such that upon operation of said heater heat willbe supplied to combustion supporting medium passed under pressure intosaid subterranean formation through said input well, passing underpressure into said subterranean formation through said input well acombustion supporting medium and operating said heater to supply heat tosaid combustion supporting medium in an amount to bring its temperatureabove the ignition temperature of said combustible material uponentering said subterranean formation, thereafter continuing passage ofsaid combustion supporting medium into said subterranean formation,continuing operating said heater to supply heat to said com bustionsupporting medium in an amount to bring its ternperature above theignition temperature of said combustible material upon entering saidsubterranean format on until the rate-pressure ratio of said combustionsupporting medium decreases, next operating said heater to reduce thesupply of heat to said combustion supporting material to an amountinsufiicient to bring its temperature above the ignition temperature ofsaid combustible material upon entering said subterranean formationuntil the rate-pressure ratio of said combustion supporting mediumincreases, thereafter operating said heater to supply heat to Saidcombustion supporting medium in an amount to 9 bring its temperatureabove the ignition temperature of said combustible material uponentering said subterranean formation until the rate-pressure ratio ofsaid combustion supporting medium decreases, and then alternatelyoperating said heater to reduce the supply of heat to said combustionsupporting medium to an amount insufiicient to bring its temperatureabove the ignition temperature of said combustible material uponentering said subterranean formation until the rate-pressure ratio ofsaid combustion supporting medium increases and operating said heater tosupply heat to said combustion supporting medium in an amount to bringits temperature above the ignition temperature of said combustiblematerial upon entering 10 said subterranean formation until therate-pressure ratio of said combustion supporting medium decreases untilsustained combustion is effected within said subterranean formation.

References Cited in the file of this patent UNITED STATES PATENTS2,382,471 Frey Aug. 14, 1945 2,390,170 Barton et a]. Dec. 11, 1945 102,584,606 Merriam et a1. Feb. 5, 1952 2,642,943 Smith et al. June 23,1953

2. A PROCESS OF INITIATING COMBUSTION OF A COMBUSTIBLE MATERIAL IN ASUBTERRANEAN FORMATION CONTAINING SAID COMBUSTIBLE MATERIAL COMPRISINGPASSING UNDER PRESSURE INTO SAID SUBTERRANEAN FORMATION THROUGH AN INPUTWELL LEADING THERETO A COMBUSTION SUPPORTING MEDIUM FOR SAID COMBUSTIBLEMATERIAL AND SUPPLYING HEAT TO SAID SUBTERRANEAN FORMATION TO EFFECTCOMBUSTION OF SAID COMBUSTIBLE MATERIAL, THEREAFTER CONTINUING PASSAGEOF SAID COMBUSTION SUPPORTING MEDIUM INTO SAID SUBTERRANEAN FORMATION,CONTINUING THE SUPPLY OF THE HEAT TO SAID SUBTERRANEAN FORMATION UNTILTHE RATE-PRESSURE RATIO OF SAID COMBUSTION SUPPORTING MEDIUM DECREASES,NEXT DISCONTINUING THE SUPPLY OF HEAT TO SAID SUBTERRANEAN FORMATIONUNTIL THE RATEPRESSURE RATIO OF SAID COMBUSTION SUPPORTING MEDIUMINCREASES, THEREAFTER CONTINUING THE SUPPLY OF HEAT TO SAID SUBTERRANEANFORMATION UNTIL THE RATE-PRESSURE RATIO OF SAID COMBUSTION SUPPORTINGMEDIUM DECREASES, AND THEN ALTERNATELY DISCONTINUING THE SUPPLY OF HEATTO SAID SUBTERRANEAN FORMATION UNTIL THE RATE-PRESSURE RATIO OF SAIDCOMBUSTION SUPPORTING MEDIUM INCREASES AND CONTINUING THE SUPPLY OF HEATTO SAID SUBTERRANEAN FORMATION UNTIL THE RATE-PRESSURE RATIO OF SAIDCOMBUSTION SUPPORTING MEDIUM DECREASES UNTIL SUSTAINED COMBUSTION ISEFFECTED WITHIN SAID SUBTERRANEAN FORMATION.